A "nacelle" is the housing over the jet engine with the forward part being a "nose cowl" made of a composite material attached to an aluminum alloy air inlet ring, which is called a "lipskin". See FIG. 1.
Lipskin rings may be up to about 3.66 meters (12 feet) in diameter. Because of the size, weight, and being attached to a composite structure, during the set up, hundreds of rivet holes are drilled and countersunk through the lipskin. During these procedures, the lipskins are invariably scratched, gouged, nicked, or otherwise damaged.
In operation, lipskins and leading edges of wings and tails are subjected to severe environmental exposure, which causes corrosion of these parts. The temperature in flight may vary from a low of -55.degree. C. to a high of 60.degree. C. Also, these parts are subjected to high velocity impact of dirt and runway debris. In flight, water droplet erosion adversely affects the lipskin and the leading edges. Additionally, these parts are subjected to temperatures of 190.degree. C. (375.degree. F.) or higher, up to 232.degree. C. (450.degree. F.), during deicing.
Thus, these leading edges are exposed to a unique combination of severe environmental conditions.
Aerospace aluminum alloys used in lipskins and leading edges are generally of the 2XXX series and contain copper, which provides strength when heat treated. Most members of the 2XXX series and other aerospace aluminum alloys, such as the 6XXX and 7XXX series, soften when exposed to temperatures used to deice a plane (between 121.degree. C. (250.degree. F.) and 190.degree. C. (375.degree. F.)). Temperatures of up to 232.degree. C. (450.degree. F.) may be used for deicing in an emergency situation.
The most common aluminum-copper alloy used for aerospace applications is AA2024, which has 4.4% copper, 1.5% magnesium, 0.6% manganese, and the remainder aluminum. AA2024 is commonly used in the leading edges of aircraft wings and the tail assembly. Leading edges are generally composed of multiple "C" shaped aluminum pieces about 8 inches across and 7-8 feet long. The leading edge may be attached by bolts or riveted to the body of the wing or tail. Leading edges are exposed to high temperatures during deicing, which causes softening of the alloy. However, such softening is generally not critical because the leading edge is not a structural member and, unlike the lipskin, is not a large self-supporting structure attached to a composite structure. The leading edges are subject to corrosion from high temperatures (deicing) and salt fog exposure and to erosion from water droplets from rain, or sleet, for example. Often, the leading edges are clad with Al 1100, pure aluminum, which does not corrode but is very soft. Because of its softness, Al 1100 scratches easily and cannot be repaired cosmetically. The rivets are ground to be flush with the clad aluminum leading edge, with great care being necessary to ensure that the thin cladding, usually about 100 .mu.m (0.004 inches) thick, is not removed in the grinding process. It requires frequent maintenance, such as frequent polishing to maintain a cosmetic appearance. Polishing, however, is very labor intensive, especially due to the presence of large numbers of rivets, and does not repair scratches cosmetically.
Because AA2219 is heat tolerant (will not lose strength) at temperatures of up to 232.degree. C. (450.degree. F.), it is the most common alloy used in lipskins of nacelles. AA2219 has 6.3% copper, 0.30% manganese, 0.34% of total of vanadium, zirconium, and titanium, and the remainder aluminum. In older jet powered air craft, lipskins were made from sections of clad aluminum alloys joined together to form a ring, which offered some protection from corrosive environmental stresses. In modern manufacture of lipskins, the lipskins are made from one piece or from 2 or more partial-circular pieces. Because of the extreme deep drawing forming stresses these one or two piece lipskins cannot be formed from clad alloys.
The present state of the art method to protect lipskins is by anodizing by sulfuric acid per MIL-A-8625, followed by sealing in boiling water or other sealant to produce a clear or aluminum finish. Generally, following anodizing, lipskins are left unpainted due to the erosion from temperature extremes experienced by the lipskins and to the difficulty in cosmetically repairing the painted surface.
Anodizing protects the lipskin from corrosion, but only for a short time. The anodic coating is very thin and does not have a long erosion life, usually lasting only a few weeks. Thus, the anodic coating protects the lipskin from corrosion during manufacture and setup procedure. However, anodizing does not afford any protection from corrosion due to in-operation environmental stresses.
In addition, damage to the lipskins that occurs during manufacture and set up also damages the anodic coating. Both the erosion of the anodic coating and damage to it from handling during manufacture leave the lipskin unprotected and subject to corrosion. During operation, corrosion is accelerated as corrosion products are washed away by air, which exposes the unprotected lipskin to the corrosive environment.
Such corrosion damage in lipskins and on the leading edges of the wings, even if only on the surface, is unacceptable to commercial airlines because these parts are visible to passengers. Therefore, airlines often will refuse delivery of nose cowls or of nacelles with damaged lipskins and leading edges of wings or will accept these parts from the manufacturer only with a cosmetic concession.
It is known that thicker anodic coatings will offer better protection than thinner anodic coatings. However, increasing the thickness of anodic coatings has not proven feasible because increasing the thickness of the anodic coating leads to reduced fatigue life of the anodized part.
Paint based protection schemes have been tried but have proven to be unacceptable due to their lack of erosion resistance, corrosion resistance, thermal stability, and cosmetic repairability.
Chemical conversion coatings per MIL-C-5541 are often used to protect these aluminum alloys, especially to repair anodic coatings. These coatings provide good room temperature corrosion resistance but do not protect in high temperatures and do not match the color of the anodic coatings and are, therefore, cosmetically unacceptable.
The problems associated with wing leading edges and tail horizontal and vertical stabilizer leading edges are somewhat different than for lipskins. The wing and tail leading edges are formed of clad aluminum alloys which are not structural. These leading edges are physically abraded to remove visible residues of the rivet heads then polished to a bright luster. Such surfaces suffer the same cosmetic matching problems as lipskins; the polished surface, if damaged, cannot be touched up and must be repolished. Since polishing consumes the soft clad layer, eventually the bare basic alloy is exposed creating more cosmetic problems and corrosion.
In spite of these differences between lipskins and leading edges, both these surfaces suffer from cosmetic matching problems in manufacture and use; all are subject to deicing temperatures (232.degree. C.) and the water droplet erosion environment described for lipskins. The basic coating requirements of the lipskins and leading edges are the same and all coatings and tests which are applicable to lipskins are applicable to leading edges, and vice versa.
Preferred characteristics of an ideal corrosion protection system for lipskins and leading edges include:
1) resistance to temperatures used in deicing, that is up to 190.degree. C. (375.degree. F.) on a routine basis and up to 232.degree. C. (450.degree. F.) on an occasional emergency basis, PA1 2) resistance to high temperatures of 300.degree. C. for 48 hours or 150.degree. C. for 100 hours. Following the 150.degree. C. exposure the coating must pass the cross-hatch adhesion test, ASTM D3359, method B (ISO 2409), and have a rating of "5" (no coating pickoff). In addition, the coating must withstand a direct impact test per ASTM D2792 at 1.38 kg-meters impact (120 inch-pounds), PA1 3) resistance to low temperatures of -55.degree. C. for one hour. The coating must show continued excellent adhesion as measured by the cross-hatch adhesion test, ASTM D3359, method B (ISO 2409), by having a rating of "5" --no pickoff-- and also resistance to ASTM D2794 Direct Impact, passing the test at 1.38 kg-meters impact (120 inch-pounds), PA1 4) resistance to corrosion as measured by the salt spray corrosion test, ASTM B117, and the filiform corrosion test, ISO 4623, PA1 5) resistance to immersion in aircraft fluids, such as aviation fuel, lubricating oil, de-icing fluid, and distilled water, PA1 6) resistance to erosion from high velocity water droplets as measured using a standardized test such as exposure to drops of a mean drop diameter of 2 mm, at an impact velocity of 223 meters/second, at an impact angle of 900, at an intensity of 25 mm/hour, for 10 minutes, and PA1 7) repairability by touching up damaged areas, so that the repaired area is essentially indistinguishable by visual inspection from adjacent non-damaged areas, herein "cosmetic" repairability. Such a repair should be invisible to 20/20 naked eye observation from a distance of 3 meters.
When confronted with these stringent and cumulative requirements, it was not known whether a coating that would meet such requirements was available. A study of the various techniques described above unfortunately did not shed useful light on the solution of the problem.
Protective metal filled phosphate bonded coatings, including those that are chromium-containing, appeared at first to be satisfactory in many ways, it was disappointing to find out in further work that it did not meet the essential criteria of cosmetic repairability of the lipskin. Further search for a single layer coating that would meet all the above stated requirements was of no avail.
Generally, in the coating field, a single layer coating that meets all the requirements is preferred over a coating of multiple layers for numerous reasons, including possible lack of compatibility or adhesion of the coatings to each other, and other potential problems. Nonetheless, it appeared to be necessary to reconcile oneself to find a suitable second coat, or top coat that would meet the repairability requirement while being free of the problems often associated with a two-layered coating. Unexpectedly, it was found that an aluminum silicone paint admirably fulfilled all the necessary requirements of compatibility with the phosphate bonded basecoat and also provided repairability.
Subsequently, it was unexpectedly discovered that other first or basecoats could be used that met the set of requirements identified herein for a suitable basecoat and further that top coats other than the aluminum silicone paint also fulfill the necessary requirements of compatibility and repairability. These coatings are described hereinafter. As a result a two layered coating was perfected that fulfilled all the requirements for lipskin and leading edges of aircraft wings.